Solvent-borne, polyester thermosetting coatings that exhibit high hardness are commercially desirable. Coatings with high hardness typically exhibit high gloss, good distinctness of image (DOI), fast dry-times, scratch, stain, chemical, and humidity resistance, and outdoor durability.
Aliphatic polyesters are frequently used to reduce viscosity and increase solids for thermosetting acrylic (abbreviated herein as “TSA”) coatings. Aliphatic polyesters also are useful as a primary film former in high solids coatings because of their ability to reduce volatile organic compound (“VOC”) emissions. Coatings made from aliphatic polyesters are generally flexible but tend to be soft, which results in poor resistance to solvents and chemicals, poor humidity resistance and poor outdoor durability.
Hardness and hardness-related properties of aliphatic polyester resins sometimes can be improved with the addition of 1,4-cyclohexanedimethanol (abbreviated herein as “CHDM”) and hydrogenated bisphenol A (abbreviated herein as “HBPA”). Unfortunately, poor solvent solubility and compatibility with acrylic resins often are observed as CHDM content is increased in the polyester resin formulation. The amount of CHDM that can be incorporated into the resin formulation, therefore is limited. HBPA is know to exhibit similar characteristics.
Poor resin solubility often manifests itself over time by phase separation, precipitation of the resin from solution, and the development of hazy to opaque resin solutions. These characteristics are undesirable and limit the storage stability of the resin solution and the coatings formulated from these solutions. Such coatings, for example, may experience a viscosity increase, phase separation, agglomeration of ingredients, etc., that result in an undesirable higher application viscosity, poor appearance and poor mechanical properties of the cured film.
Thermosetting acrylic (“TSA”) resins are widely used in industrial coatings. They can be formulated to a high glass transition temperature (Tg) and exhibit excellent light stability and hydrolysis resistance. These properties result in coatings with the desirable characteristics of high hardness; fast dry times; resistance to stains, chemicals and humidity; and good outdoor durability. Consequently, TSA resins often serve as the primary film-former in coatings for demanding applications that include transportation, maintenance, marine and building/construction markets.
Although TSA resins exhibit many desirable properties, they often lack flexibility and require more solvent in the coating formulation to achieve a practical application viscosity. The higher solvent requirement for TSA resins makes it difficult to achieve high solids coatings with reduced VOC content as mandated by various federal and state air quality organizations.
To increase resin and coating solids, reduce viscosity, and lower VOC emission, aliphatic polyester resins can be blended with TSA resins. Unfortunately, the glass transition temperature (“Tg”) of the blend frequently decreases significantly as the polyester content increases. The lower Tg of the blend has an adverse impact on the desirable characteristics that the TSA resin imparts to the coating.
Automotive coatings have gradually shifted from one layer monocoat system after primer to two layer system—basecoat and clearcoat—for an improved appearance and improved durability. However, environmental compliance has resulted in the paint industry attempting to move towards greener and more “eco-friendly” products with similar performance attributes. Increases in the total solids content or replacement of certain organic solvents with water are two alternatives available to limit the amount of volatile organic compounds (VOC) in the paint formulation. Consolidated paint processes which eliminates or shorten bake and flash time in attempt to reduce energy consumption and operating cost. Accordingly, monocoat systems have found renewed interest by industry. A monocoat paint system reduces painting time and energy use by cutting the number of paint applications from three to two and the number of drying procedures from two to one. The reduction in paint and energy consumed means reduced carbon dioxide and particulate emissions compared with conventional paint processes.
There is a need in the coatings industry for aliphatic polyesters that exhibit good hardness with good flexibility and solubility when formulated into thermosetting coatings compositions. In addition, there is a need for aliphatic polyester resins that, when blended with TSA resins, lower the viscosity while maintaining the Tg of the TSA resin in high solids, thermosetting coating compositions.